Paget's Disease of bone is a condition in which rapid bone formation occurs, altering the strength and shape of the bone. Predisposition to familial Paget's Disease has been linked to a number of loci, including the Sequestosome 1 (SQSTM1) locus where germline mutations have been identified in 40% of the familial cases. Germline SQSTM1 mutations have also been found in some apparent sporadic cases of Paget's Disease, suggesting that these may be de novo familial cases. In all but one case, the constitutional mutations have been found to be heterozygous. This suggests that the mutation may be acting in a dominant manner. However, it is also possible that the mutation is acting as a dominant negative or haploinsufficiency mutation, or as a recessive mutation that has undergone loss of heterozygosity in the affected bone. We were curious as to the nature of the mutation in the affected bone. We chose to examine the SQSTM1 locus in the affected bones of sporadic patients to see: 1) whether somatic mutations in SQSTM1 occurred in these bones and 2) whether the mutation was heterozygous or homozygous in the affected bone. When we sequenced the SQSTM1 gene in both the affected bone and matched peripheral blood, we found a P392L mutation in 4 of 5 samples of affected bone and none of the matched blood samples. What was more surprising was that in the affected bone, the relative frequency of the P392L mutation in the samples ranged from 10% to 28%, suggesting that the bone was mosaic for the mutation. The relative frequency of the mutation in each individual sample was consistent upon repeated resampling and resequencing. Next, since osteosarcoma is a complication of Paget's Disease, we were curious to see whether SQSTM1 mutations were present in pagetic osteosarcomas. When we screened the SQSTM1 gene in these tumors, we found that 4 of 5 osteosarcomas had homozygous P392L mutations. Again, no mutations were detected in the matched normal bone. Together, this evidence causes a paradigm shift in our model of Paget's Disease. It suggests that while mutations in SQSTM1 are predisposing in familial Paget's Disease, they are not the initiating event in sporadic Paget's Disease. Secondly, this suggested that the tumors arose from osteoblastic cells that contained the SQSTM1 mutation and that osteoblasts play a role in the etiology of Paget's Disease. Our hypothesis is that mutations in SQSTM1 occur somatically in a subset of osteoblast-like cells in pagetic bone and that these somatic mutations become homozygous during pagetic osteosarcoma tumorigenesis. To test this hypothesis, we propose the following specific aims: 1) To test whether somatic SQSTM1 mutations are heterozygous and clonal in the pagetic bone. 2) To test whether loss of heterozygosity at the SQSTM1 locus occurs in the pagetic bone or during pagetic tumorigenesis. [unreadable] [unreadable] [unreadable]